Dorzolamide is chemically termed as (4S,6S)-4-(ethylamino)-5,6-dihydro-6-methyl-4H-thieno[2,3-b]thiopyran-2-sulfonamide 7,7-dioxide hydrochloride. Dorzolamide hydrochloride is represented by following structural Formula I:

Dorzolamide hydrochloride is known to be a carbonic anhydrase inhibitor useful in the treatment of ocular hypertension.
A process for the preparation of dorzolamide and its derivatives was first described in EP 0296879. The process of particular relevance is depicted in scheme 1.

The process disclosed in scheme 1 has following disadvantages.
(a) The reduction of the ketone of sulfonamide (vi) using absolute ethanol is carried out at reflux and then stirred at room temperature for several hours to complete the reaction. This longer duration of reaction produces many impurities.
(b) Oxidation of alcohol (vii) to sulfone (viii) is carried out using oxone. The oxone has many disadvantages such as it is irritating to the eyes, skin, nose and throat. It should be used with adequate ventilation and exposure to its dust should be minimized. Traces of heavy metal salts catalyze the decomposition of oxone. It is practically insoluble in all organic solvents hence a phase transfer catalyst is required.(c) Activation of the 4-hydroxy group of the sulfoaminated hydroxysulfone (viii) and nucleophilic substitution by desired ethylamine, results in all diastereomeric products (x) i.e. trans and cis isomers, which must be separated by column chromatography and resolved, further using resolving agent. As a result, product loss is greater when the desired product is the more active enantiomer.
An alternate route for the preparation of dorzolamide hydrochloride by the Ritter reaction is disclosed in EP0296879 and consists of the treatment of a aliphatic hydroxyl with a nitrile and a strong acid to form an amide. The process disclosed is as depicted in Scheme 2.

The reaction involves conversion of hydroxysulfones (viii) to the corresponding acetoamidosulfones (ix-a) with retention of configuration followed by reduction of the amido group, chromatographic separation and resolution to obtain the desired trans isomer (I).
The prior art teaches the use of an excess quantity of sulfuric acid to carry out the Ritter reaction and hence a large quantity of ice is required for quenching the reaction mass. When the reaction mass in concentrated sulfuric acid comes into contact with ice, a large amount of localized heat is generated causing decomposition of material. Since a huge amount of water is required for quenching the reaction mass, the amount of ethyl acetate required for extraction is also substantially large. The work-up using water is not advisable nor applicable industrially.
U.S. Pat. No. 5,688,968 describes an alternative route of preparation of dorzolamide hydrochloride starting from chiral 5,6-dihydro-4-(S)-hydroxy-6-(S)-methyl-4H-thiopyran-7,7-dioxide, as depicted in Scheme 3:

The process described in Scheme 3 has the following disadvantages:
(a) Use of expensive chiral hydroxysulfone starting material. The process for the preparation of the chiral hydroxysulfone starting material is disclosed in U.S. Pat. Nos. 5,157,129, 5,474,919 and 5,760,249. In these processes, the chiral hydroxysulfone is obtained by the asymmetric enzymatic reduction of the corresponding ketosulfone, or by cyclization of the chiral thienyl thiobutyric acid, obtained, in turn, from a chiral hydroxyester or lactone, and the subsequent stereospecific reduction of the resulting ketone.(b) The process according to this patent uses maleic acid to separate the undesired cis isomer from dorzolamide. However this maleate salt formation to remove the cis isomer is only suitable when the ratio of trans/cis is greater than 95:5. That means, the maleate salt formation of dorzolamide does not the remove cis isomer exclusively when the cis isomer content is more than 5%. It sometimes requires repeated purification to achieve the desired chiral purity.
Another alternate route for the preparation of dorzolamide hydrochloride is disclosed in U.S. Pat. No. 7,109,353 which involves the use of sodium perborate as an oxidant, as depicted in Scheme 4.

The process disclosed in Scheme 4 has following disadvantages
(a) Conversion of (i) to (ii) requires the mixture to be refluxed for 18-20 hrs which is time consuming and may cause impurity in the product.
(b) As the process uses the Ritter reaction to convert (vi) to (vii), a large amount of water is required to quench the hot mass of reaction which is not practical in an industrial set-up.
(c) Sodium perborate is used as an oxidizing agent to convert (v) to (vi), which has got bleaching properties, and the handling of it may be injurious when done so for a prolonged period.
Yet another process for the preparation of dorzolamide is disclosed in United States publication no. 20060155132 which involves protecting the chiral 5,6-dihydro-4-(R)-hydroxy-6-(S)-methyl-4H-thieno-[2,3-b]thiopyran-7,7-dioxide as depicted in Scheme 5.

The process disclosed in Scheme 5 has the following disadvantages.
(a) The conversion process of compound (II) to (III) requires a very low temperature which ranges from −30° to 0° C.
(b) The amination process requires 16-20 hrs, which is time consuming and may cause impurity in the product.
All these disadvantages of the prior art are overcome by the process in accordance with the present invention.